In many applications, data is sent in a serial manner from a transmitting device to a receiving device. Often, the data originates in the transmitting device as parallel data. Accordingly, in order to convert parallel data into serial data for transmission, the transmitting device typically includes a serializer. The serializer, in turn, typically includes a multiplexer with inputs adapted to receive the parallel data, an output adapted to produce the serial data, and a clock input adapted to receive a clock signal for clocking out the serial data. Typically, the serializer includes a phase locked loop (PLL) module adapted to generate the clock signal for clocking out the serial data based on a clock signal associated with the parallel data.
At the receiving device, a deserializer is typically employed to receive the serial data and convert it back into parallel data. The deserializer typically includes a phase locked loop (PLL) circuit to recover and generate a clock signal associated with the serial data, since the data is often transmitted without an associated clock signal. The deserializer further includes a de-multiplexer including an input to receive the serial data, outputs to produce the parallel data, and a clock input to receive the recovered clock signal in order to clock the serial data at the input to the parallel outputs of the deserializer.
In many applications, the clock signal that drives the serializer has a substantially fixed frequency so that the serial data is transmitted to the receiving device at a substantially constant rate. However, the constant rate of the serial data produces relatively high energy approximate the data rate frequency, which may leak from the system and cause electromagnetic interference (EMI). To combat EMI, specialized wired mediums, such as shielded twisted pair or coaxial cable, are often employed between the transmitting and receiving devices in order to substantially reduce EMI. However, typically such specialized wired medium are expensive and consume a relatively large footprint. Moreover, even with shielded twisted pair cable, the EMI of the system might be undesirably high, mostly due to the transmitter and receiver housings not being shielded well due to cost and design reasons.
Another technique of reducing EMI is to spread the frequency spectrum of the transmitted serial data. That is, instead of clocking the serializer with a substantially fixed frequency clock signal, the serializer is clocked with a modulated or spread-spectrum clock signal. In this manner, the energy of the transmitted serial data is no longer concentrated at substantially a single frequency, but spread over a defined frequency range. This lowers the energy of the signal at a given frequency, which, in turn, leads to reduced EMI.
Often, the serializer/deserializer pair are designed for general purposes. Accordingly, they are designed to operate over a relatively wide frequency range or data rates. Thus, the clock signal driving the serializer may have a relatively wide frequency range. The frequency of the modulation applied to the clock signal for spread-spectrum purposes is typically configured to vary as a function of the frequency of the clock signal. Accordingly, if the frequency of the clock signal varies substantially, then the frequency of the modulation may also vary substantially. This may have adverse effects in the case, for example, where the modulation frequency falls into an audio band which may produce audio interference, or rises too high for a downstream device to be able to recover the clock signal.